NORMAN, Okla. -  Oklahoma senior 141-pounder Michael Lightner was selected Big 12 Wrestler of the Week as announced by the conference office Tuesday.  He became the only Big 12 wrestler to earn the honor twice this season.    Lightner, undefeated at 30-0 this season, defeated No. 16 Mike Castillo by major decision, 14-5, in OU's 30-6 win against 12th-ranked Michigan State.  The Marlow, Okla., native is one of three Big 12 Wrestlers - Iowa Stateíss Cael Sanderson and Reggie Wright of Oklahoma State are the others - to sport a perfect record this season.    Lightner joins fellow senior Waymon May (197 pounds) and junior Michael Barger (174) who also won the award this season.  An OU wrestler was honored as Big 12 Wrestler of the Week four times this season, more than any other school.    The sixth-ranked Sooners finished the regular season with an 18-5-1 record.  OU will return to action this weekend when it travels to Stillwater, Okla., on March 2-3 for the Big 12 Wrestling Championships.